1. Field of the Invention
The present invention relates to an improvement of a table mechanism for lifting and lowering a table top by use of a parallel link mechanism.
2. Description of the Prior Art
Turning to FIG. 4, there is illustrated a typical example of a conventional table mechanism for lifting and lowering a table top through a parallel link mechanism. FIG. 4 is a view showing a concept of the table mechanism for use with a computerized tomograph. Referring again to FIG. 4, a table mechanism 1 is arranged such that a table top 2 is supported on a link mechanism 5 composed of two parallel links 3 and 4 parallel with each other. The table mechanism 1 is so installed as to be substantially orthogonal to a front face of a gantry 6 in the longitudinal direction. One ends of the two links 3 and 4 are rotatably fitted to a base 7, while the other ends thereof are rotatably fitted to a table top supporting unit 8. The link mechanism 5 driven by a driving mechanism 9, with its inclination being varied, behaves to lift and lower the table top while being held in parallel with a base 7. Solid lines and one-dotted lines respectively indicate two kinds of states of the table top 2, the link mechanism 5 and the driving mechanism 9, which are developed as a result of such moving operations. More specifically, the solid lines indicate a state of the table mechanism 1 (hereinafter referred to as an imaging position) when inserting a cradle 10 into an gantry opening 11, while the one-dotted lines show a state of the table mechanism (hereinafter referred to as an initial position) when mounting a subject for examination. The table top 2 is situated lowest in the initial position, which is preferably as low as possible to facilitate mounting of the subject. Side surfaces of the link mechanism are covered with an adequate cover in terms of security and appearance. FIG. 5 is a block diagram depicting a conventional example of a cover for covering the side surfaces of the link mechanism. The cover is composed of a plurality of cover pieces 12 and fitting plates 13 and 14, provided on the table top and on the base, for rotatably securing both ends of the cover pieces. Each of the cover pieces 12 assumes a substantially rectangular shape having a nearly equal length to that of the link 3 or 4. Spacings at which the cover pieces 12 are secured are set taking widths thereof into consideration. Namely, the spacings are set so that the cover pieces adjacent to each other cover the side surfaces of the parallel link mechanism 5 with no gap while being invariably overlapped with each other. As a result, the side surfaces of the parallel link mechanism 5 are covered with the cover pieces 12 in the imaging position or in the initial position. Turning to FIG. 6, there is schematically illustrated an array of the cover pieces 12. To be specific, the rectangular cover piece 12 having a width W is secured in such a manner that a stopping part 12a conceived as a corner of the rectangle is spaced a distance L away from a base surface 15. The distance L is so set that a corner 12c of the cover piece 12 does not touch the base surface 15. As is obvious from the Figure, if the initial position is made lower than before at the thus set distance L, it is required that the cover piece width W be smaller than in the previous case. If the cover piece width W is diminished, a greater number of cover pieces are needed for covering the side surfaces of the link mechanism. Consequently, there arises a problem of being costly due to an increase in the number of cover pieces when actualizing a device incorporating a link mechanism providing a lower initial position.